The line integral of the magnetic flux around a closed curve isproportional to the algebraic sum of electric currents flowingthrough that closed curve. This was later modified to add a second term when it wasincorporated into Maxwell's equations.

Anthropic principle

Weak anthropic principle. The conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. That is, the region of the Universe in which we live is not necessarily representative of a purely random set of initial conditions; only those favorable to intelligent life would actually develop creatures who wonder what the initial conditions of the Universe were.

Strong anthropic principle. A more forceful argument that the weak principle: It states, rather straightforwardly, that if the laws of the Universe were not conducive to the developmentof intelligent creatures to ask about the initial conditions of the Universe, intelligent life would never have evolved to ask the question in the first place. In other words, the laws of the Universe are the way they are because if they weren't, you would not be able to ask such a question.

Arago spot (D.F.J. Arago)

A bright spot that appears in the shadow of a uniform disc beingbacklit by monochromatic light emanating from a point source.

Archimedes' principle

A body that is submerged in a fluid is buoyed up by a force equalin magnitude to the weight of the fluid that is displaced, anddirected upward along a line through the center of gravity of thedisplaced fluid.

Atwood's machine

A weight-and-pulley system devised to measure the acceleration dueto gravity at Earth's surface by measuring the net acceleration ofa set of weights of known mass around a frictionless pulley.

Avogadro constant; L; NA (Count A. Avogadro; 1811)

The number of atoms or molecules in a sample of an idea gas whichis at standard temperature and pressure. It is equal to about6.022 52.1023 mol-1.

Avogadro's hypothesis (Count A. Avogadro; 1811)

Equal volumes of all gases at the same temperature and pressurecontain equal numbers of molecules. It is, in fact, only true forideal gases.

Balmer series (J. Balmer; 1885)

An equation which describes the emission spectrum of hydrogen whenan electron is jumping to the second orbital; four of the linesare in the visible spectrum, and the remainder are in theultraviolet.

Baryon decay

The theory, predicted by several grand-unified theories, that aclass of subatomic particles called baryons (of which the nucleons-- protons and neutrons -- are members) are not ultimately stablebut indeed decay. Present theory and experimentation demonstratethat if protons are indeed unstable, they decay with a halflife ofat least 1034y.

Bernoulli's equation

An equation which states that an irrotational fluid flowingthrough a pipe flows at a rate which is inversely proportional tothe cross-sectional area of the pipe. That is, if the pipeconstricts, the fluid flows faster; if it widens, the fluid flowsslower.

BCS theory (J. Bardeen, L.N. Cooper, J.R. Schrieffer; 1957)

A theory put forth to explain both superconductivity andsuperfluidity. It suggests that in the superconducting (orsuperfluid) state electrons form Cooper pairs, where two electronsact as a single unit. It takes a nonzero amount of energy tobreak such pairs, and the imperfections in the superconductingsolid (which would normally lead to resistance) are incapable ofbreaking the pairs, so no dissipation occurs and there is noresistance.

Biot-Savart law (J.B. Biot, F. Savart)

A law which describes the contributions to a magnetic field by anelectric current. It is analogous to Coulomb's law forelectrostatics.

Blackbody radiation

The radiation -- the radiance at particular frequencies all acrossthe spectrum -- produced by a blackbody -- that is, a perfectradiator (and absorber) of heat. Physicists had difficultyexplaining it until Planck introduced his quantum of action.

Bode's law

A mathematical formula which generates, with a fair amount ofaccuracy, the semimajor axes of the planets in order out from theSun. Write down the sequence 0, 3, 6, 12, 24, . . . and then add4 to each term. Then divide each term by 10. This is intended togive you the positions of the planets measured in astronomicalunits.

Bode's law had no theoretical justification when it was firstintroduced; it did, however, agree with the soon-to-be-discoveredplanet Uranus' orbit (19.2 au actual; 19.7 au predicted).Similarly, it predicted a missing planet betwen Mars and Jupiter,and shortly thereafter the asteroids were found in very similarorbits (2.8 au actual for Ceres; 2.8 au predicted). However, theseries seems to skip over Neptune's orbit.

Bohr magneton (N. Bohr)

The quantum of magnetic moment.

Bohr radius (N. Bohr)

The distance corresponding the mean distance of an electron fromthe nucleus in the ground state.

Boltzmann constant; k (L. Boltzmann)

A constant which describes the relationship between temperatureand kinetic energy for molecules in an ideal gas. It is equal to1.

Boyle's law (R. Boyle; 1662); Mariotte's law (E. Mariotte; 1676)

The product of the pressure and the volume of an ideal gas atconstant temperature is a constant.

Brackett series (Brackett)

The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the fourth orbital. All of the linesare in the infrared portion of the spectrum.

Bragg's law (Sir W.L. Bragg; 1912)

When a beam of x-rays strikes a crystal surface in which thelayers of atoms or ions are regularly separated, the maximumintensity of the reflected ray occurs when the sine of thecompliment of the angle of incidence is equal to an integermultiplied by the wavelength of x-rays divided by twice thedistance between layers of atoms or ions.

Brewster's law (D. Brewster)

The extent of the polarization of light reflected from atransparent surface is a maximum when the reflected ray is atright angles to the refracted ray.

Brownian motion (R. Brown; 1827)

The continuous random motion of solid microscopic particles whensuspended in a fluid medium due to the consequence of continuousbombardment by atoms and molecules.

Carnot's theorem (S. Carnot)

The theorem which states that no engine operating between twotemperatures can be more efficient than a reversible engine.

centrifugal pseudoforce

A pseudoforce -- a fictitious force resulting from being in a non-inertial frame of reference -- that occurs when one is moving inuniform circular motion. One feels a "force" outward from thecenter of motion.

Chandrasekhar limit (S. Chandrasekhar; 1930)

A limit which mandates that no white dwarf (a collapsed,degenerate star) can be more massive than about 1.2 solar masses.Anything more massive must inevitably collapse into a neutronstar.

Charles' law (J.A.C. Charles; c. 1787)

The volume of an ideal gas at constant pressure is proportional tothe thermodynamic temperature of that gas.

Cherenkov radiation (P.A. Cherenkov)

Radiation emitted by a massive particle which is moving fasterthan light in the medium through which it is travelling. Noparticle can travel faster than light in vacuum, but the speed oflight in other media, such as water, glass, etc., are considerablylower. Cherenkov radiation is the electromagnetic analogue of thesonic boom, though Cherenkov radiation is a shockwave set up inthe electromagnetic field.

Complementarity principle (N. Bohr)

The principle that a given system cannot exhibit both wave-likebehavior and particle-like behavior at the same time. That is,certain experiments will reveal the wave-like nature of a system,and certain experiments will reveal the particle-like nature of asystem, but no experiment will reveal both simultaneously.

Compton effect (A.H. Compton; 1923)

An effect that demonstrates that photons (the quantum ofelectromagnetic radiation) have momentum. A photon fired at astationary particle, such as an electron, will impart momentum tothe electron and, since its energy has been decreased, willexperience a corresponding decrease in frequency.

Coriolis pseudoforce (G. de Coriolis; 1835)

A pseudoforce -- a fictitious force, like the centrifugal "force"-- which arises because the rotation of the Earth varies atdifferent latitutdes (maximum at the equator, zero at the poles).

correspondence principle.

The principle that when a new, more specialized theory is putforth, it must reduce to the more general (and usually simpler)theory under normal circumstances. There are correspondenceprinciples for general relativity to special relativity andspecial relativity to Newtonian mechanics, but the most widelyknown correspondence principle (and generally what is meant whenone says "correspondence principle") is that of quantum mechanicsto classical mechanics.

Cosmic background radiation; primal glow

The background of radiation mostly in the frequency range 3.1011 to 3.108 Hz discovered in space in 1965. It is believedto be the cosmologically redshifted radiation released by the BigBang itself. Presently it has an energy density in empty space ofabout

Cosmological redshift

An effect where light emitted from a distant source appearsredshifted because of the expansion of space itself. Compare withthe Doppler effect.